In general, the human ear is divided into three primary parts: outer ear, middle to ear and inner ear. The sound vibration occurring outside the ear is directed into an earcanal of the outer ear by pinna, travels along the earcanal and arrives at the tympanic membrane (eardrum). The earcanal is a kind of a resonance tube the end of which is closed by the eardrum. The vibration of the eardrum is transmitted to the inner ear through three small bones in the middle ear, namely, malleus, incus, and stapes. When the vibrations of the ossicles are transmitted to cochlea through the footplate of the stapes, endolymph inside the cochlea moves, and thousands of tiny hair cells in the scala media in the cochlea moves as well accordingly, the result of which is then conversion of the acoustic stimuli into electrical signals. Then the electrical signals are transmitted to the brain through the central nervous system, and sound perception occurs. Hearing loss, which requires wearing of a hearing aid, is classified into three types: conductive hearing loss, sensorineural hearing loss and mixed hearing loss. Although all the three types could be alleviated with hearing aids, persons with sensorineural hearing losses are the usual candidates for hearing aids. Persons with conductive hearing loss are usually treated with medicine or surgery. And those with mixed losses are medically and surgically treated first and then hearing aids are recommended.
Hearing aids are classified into three major types: pocket type, behind-the ear (BTE) type and in-the-ear (ITE) type. The ITE type is subdivided further according to the depths of insertion and sizes. Recently, the ITE types are the preferred ones due to its less conspicuous appearance and to less cumbersome aspect. In order to prevent from further worsening of hearing loss through over-amplification and to assure clearer speech perception ability, hearing aids should be fitted optimally individually. Here, the phrase ‘hearing aid fitting’ is used to mean a process of selecting an appropriate hearing aid according to the audiogram of hearing impaired person, periodically checking the hearing aid performances such that the gain and SSPL of each frequency band of the hearing aid are accurately tuned, thereby monitoring the hearing aid performance so as to be used without malfunction.
However, all the conventional hearing aid fitting methods, either utilize criteria of indirectly standardized values of a so-called insertion gain or 2 cc coupler gain, or adjust according to wearer's subjective judgment, as shown in FIG. 1. The insertion gain is the average difference between unaided and aided gain, and the 2 cc coupler gain is the average gain standardized to normal earcanal volumes of Caucasian adults without wearing earmolds connected to hearing aids. That is, the adjustments of the gains and SSPLs of hearing aids are not made to the amplified sounds in front of tympanic membranes with hearing aids on, so that various variables concerning different earmolds, different sizes of earcanals, location of hearing aid microphone and so on affect differently in each individual, the result of which is that the criteria might be meaningless in most patients. Referring to FIG. 1, for example, according to the conventional hearing aid fitting method, patient's identification (ID) and audiogram are entered, a couple of hearing aid conditions, i.e., the type of earmold and the type of hearing aid are selected and then a ‘best fit’ button is pushed (steps S101 through S104). When the ‘best fit’ is activated, criterion values for gains and SSPLs obtained by the 2 cc coupler average values are determined in each frequency band irrespective of the various states and conditions of individuals. The patient then wears hearing aids on and signals of the amplitudes of 50 dB SPL (for gain) are swept throughout the whole frequency range, and the wearer reports whether stimuli of each frequency band are equally loud or not. If not, the discrepant portions are corrected (steps S105 through S107). And then, stimuli of 90 dB SPL (for SSPL) are swept and the same subjective corrections are made.
Since conventional hearing aid fitting methods adopt average criterion values based on 2 cc coupler measurements or insertion gain, which are not standardized on individually measured output values of hearing aid in front of the eardrum, individual variables of size of external earcanal, shape of earmolds, location of the microphone of hearing aid, the state of pinna and the like are not taken into consideration, so that it is impossible to achieve accurate individual fitting so as to be tailored to the individual user. Also, the sweeping procedure not only requires a longer fitting time, but also is an inaccurate method, since subjective judgment is inaccurate frequently. Therefore, several revisits for readjustment are a routine procedure. Furthermore, since the user's subjective cooperation is required in fitting procedure, conventional procedures are incapable methods to apply in cases of infants or elderly persons.